1. Field of the Invention
This invention generally relates to a method for forming metallic coat or layer on the surface of workpiece. More concretely, this invention relates to a method for forming metallic coat which can be applied in various kinds of technical field, e.g. forming of printed circuit, ceramic, ceramics, ceramic product, metal tableware, several kinds of thermosetting resin molding product, surface decoration for a paper and the like having at least enough thickness to allow the ejection of metal powders without making a through hole thereon.
Further, this invention relates to a method for forming metallic coat on the surface of workpiece at its predetermined part, and for nitriding in general meaning including soft nitriding the surface of workpiece at its rest part, by coating the above predetermined part with the metallic coat formed by the above method, before nitriding, so that the above coated predetermined part is prevented from being subjected to nitriding.
2. Description of the Prior Art
In many technical fields, it is common to form metallic coat having a predetermined pattern on the surface of workpiece. For example, on the surface of ceramic product, metallic coat made of gold, silver and the like is formed with a predetermined pattern as surface decoration. As another example, on the surface of ceramic substrate, metallic coat is formed with a predetermined pattern for forming a printed circuit.
For forming the metallic coat in the above variety of technical fields, there are several methods. For example, there is a method comprising: forming metallic coat on the surface of workpiece by some operations including first painting, electroplating and the like; and reacting the metallic coat with chemicals so that abrasion occurs there for etching with a predetermined pattern. Alternatively, there is another method comprising: painting a paste, which contains metallic powders as coating material and solvent, on the surface of workpiece with a predetermined pattern by screen process printing, dipping the workpiece after masking, transcripting with a transfer paper, or the like; melting the metallic powders contained in the paste by heating this paste and fixing the metallic powders to the surface of workpiece so that metallic coat with a predetermined pattern is formed; washing the workpiece so that the solvent which has been emerged due to heating is removed; and obtaining a product on which the metallic coat having the pattern according to the shape of the above painted paste was formed. Addition to these methods, a vacuum deposition method also can be used.
In the above conventional methods, by the method where the metallic coat is formed by merely painting, adhesion is too low. Further, by the other conventional method where the metallic coat formed by electroplating or the like is subjected to etching, several kinds of chemicals are required during the etching, and long time and high cost are required for forming the coat and for the etching. By the vacuum deposition method, although high adhesion can be obtained, high cost is required like baking method. Further, the utilization of products obtained by this vacuum deposition method is limited.
On the other hand, by the conventional method where the coat is formed by painting the above paste containing the above metallic powders, the pattern of formed metallic coat can be determined during painting of paste, i.e., the predetermined pattern can be formed without any process such as etching. However, in order to obtain the predetermined pattern precisely, this conventional method requires other many processes such as painting of paste, fixing of metallic coat by means of heating, washing of solvent and the like. Further, when the metallic coat is fixed due to heating as stated before, the solvent contained in the paste and metallic powders are partly evaporated and released in the air, resulting in that the working environment becomes worse. Particularly, when solder coat is formed, since lead contained in the solder is evaporated, a problem is caused. Precisely, if large amount of evaporated lead is breathed by a human, it is feared that his or her health condition might be damaged. Accordingly, if metallic coat is formed by such method, the working environment must be carefully prepared.
Additionally, in order to retain the metallic powders just in the pattern of paste painted on the surface of workpiece, the various conditions, e.g., the viscosity of paste, the kind of solvent, the particle size of metallic powder, temperature to which the paste is heated, must be determined optimally. However, this determination is so difficult.
Moreover, when the solvent and the equivalents, which have been emerged on the surface of workpiece, are washed, a washing agent containing many kinds of chemicals is required. Further, after washing, the washing agent contains the metallic powders and solvent, which have been removed from the surface of workpiece by washing, hence, such washing agent can not be discharged into river and the like as it is. Therefore, facilities must be provided in order to clean and neutralize the agent, resulting in a problem.
A problem which is common in the foregoing conventional methods is as follows. Precisely, products particularly ones made of ceramics, on each surface of which metallic coat having a predetermined pattern is formed by the conventional methods, lose their inherent mechanical strength against usual utilization, usual use, repeated washing and the like. Additionally, each product does not have sufficient peel strength (particularly, water resistance peel strength). Further, it has been known that stress corrosion often occurs on ceramics. Then, since a glass layer is included in the ceramics as a grain boundary layer, stress corrosion is caused by not only acid and alkali but also moisture. This stress corrosion progresses so quickly that the ceramics is affected by even vapor in the air. Also in a crystalline having an Al--O bond, stress corrosion caused from moisture is found as "Fatigue" or "Slow Crack Growth".
Engraving as decoration on each conventional glass product has been performed typically by cutting with grinder or the equivalent, sandblasting, etching with chemicals, or the like. However, in every case, since irregularity remains as cracks on the surface of workpiece, the glass product is easily broken by increasing stress corrosion caused from external factors.
There are some metallic work materials, in each of which, it is required that the part of its surface is subjected to nitriding, but the other part of surface is not subjected to nitriding so as to be remained as it is. Accordingly, in order to prevent nitrogen from dispersing at the part where the nitriding is not required, this part is plated before the nitriding. By doing so, this plated part is prevented from being subjected to nitriding. There are several methods for preventing the nitriding in this way.
In these methods, there is a method where hot-dipping is used for preventing the nitriding. In this method, the work material should be dipped in heated and melted metal, thus, such operation is dangerous. Additionally, the excessive amount of metal adhered to the surface of work material must be removed, which requires long time.
Further, heated and melted metal is required for the dipping, thus, a facility for heating the metal is necessary in order to maintain its melting state. This causes the problem of high cost.
Moreover, since the adhesion of plating layer formed by using the hot-dipping is small, the plating layer is easily peeled from the surface of work material. Accordingly, many substandard products are produced. This is the reason why the cost of each product formed by using the hot-dipping is increased.
Alternatively, there is a method where electroplating is used for preventing the nitriding. However, this method requires long time until the metal is deposited on the surface of work material and plating is formed. Further, if the plating is required to be fine, electro-deposition should be performed at low voltage. Accordingly, this electroplating method requires much longer time. That is to say, due to the long plating operation, this method has the problem of high cost.
Then, when welding or the like is performed on the work material which has been subjected to nitriding, the plating must be peeled at parts where the welding is to be performed. However, if the plating is made of nickel, this plating formed by the above method using electroplating can not be peeled easily from the surface of work material. Therefore, a release agent such as caustic soda is required. Accordingly, the facility for treating the release agent is necessary, resulting in high cost for its maintenance and management.
The present invention has been developed for solving the problems existing in the above conventional technique. The object of the present invention thereby is to provide a metallic coat forming method, by which no washing process for solvent or the like is required by needlessness of such solvent or the like in order to form the metallic coat ; by which work environment and peripheral environment can be prevented from becoming worse by needlessness of heat process or the like in order to form the metallic coat having high adhesion; and by which the metallic coat with a predetermined pattern can be easily formed at low cost with a relatively simple equipment or facility.
Another object of the present invention is to provide a metallic coat forming method particularly for prevention of nitriding. In this method, the metallic coat, which has high density as well as high strength of adhesion to the surface of work material and which can be easily peeled from the surface of work material after nitriding, is formed on the surface of work material at its part where the nitriding should be prevented, so that the nitrogen can be surely prevented from dispersing in the part on which the metallic coat has been formed, during the nitriding of work material.